"Grandmother and grandfather"
Translation:Бабуся та дідусь
In my dialect of English, no-one would use any of the examples which you suggest: they are way too "American". I don't know if the syntax in which Duolingo "accepted answers" are entered has a compact way in which to express numerous alternatives (for example "mum|mom|mummy|mommy and dad|daddy". If it doesn't, they need to make that possible, so that course authors are not overwhelmed by having to express numerous alternatives in full.
But "That's complicated" isn't even half a reason to shy away from doing it right.